In recent years, there have been employed a cleaning method and a cleaning apparatus, to which ultrasonics of less than 1 MHz (e.g. 500 kHz), so-called the megasonics is applied, for precision cleaning of a silicon substrate and a liquid crystal substrate. That is because, when megasonics is employed, particulate (especially, particulate 0. 2 .mu.m or less) on a substrate can effectively be removed by cleaning with a low-density cleaning liquid (chemical liquid) under the room temperature.
However, a frequency of ultrasonics effects to cleaning efficiency and damage to the substrate itself. It is known that, when ultrasonics of 500 kHz or less is used for precision cleaning, for example, a substrate with a pattern such as a wiring pattern thereon is easily damaged due to the phenomenon of erosion by the ultrasonics.
On the other hand, it is generally recognized that a higher frequency gives less damage to a substrate, and that cleaning effects to a minute spot thereon and uniformity of cleaning over the entire substrate are improved.
With the current state of the art, therefore, a frequency of around 500 kHz is used to be brought into balance among effective removal of particulate from a substrate, prevention of damage to the substrate, and uniformity in the cleaning.
It is desired, however, that megasonics with a much higher frequency is used to satisfy the needs for improvements in making a pattern finer (for a distance between each wiring of 0. 5 .mu.m or less) and in cleaning uniformity over a substrate having a large area (e.g. an 8-inch wafer).
There is still remaining the problem that removal efficiency of particulate is not sufficient in an area of a frequency exceeding 1 MHz. Also, it is found that, when cleaning is performed actually using ultrasonics with a frequency of 1 MHz or more, uniformity in cleaning is not always improved and lack of uniformity in cleaning occurs, and in addition some flaws are made on the surface of the pattern due to an uncertain cause.
It is an object of the present invention to provide a cleaning method which is excellent in removal efficiency of minute particles and can clean a substrate even with a large area with excellent uniformity in cleaning the substrate without giving any damage to the substrate.